Hardware Shader Implementation

Hardware shader names can be explicitly specified in materials, or automatically constructed from software shader names by adding a vendor prefix like migl_. Once the shader name is known, mental ray will perform two operations:

• First, mental ray will attempt to call a software function whose name is constructed by adding the prefix to the shader name. For example, if the hardware shader name is myphong and the hardware shader is implemented in Cg, mental ray will try to call the software function micg_myphong. This is not a hardware or software shader but a C/C++ function that can be used to do special setup operations needed for the shader, such as copying data to the graphics board or calling special GL functions needed by the shader. This function is optional and rarely necessary.

  This function is normally implemented in the shader support library. For example, the software shader library base.so might have a support library migl_base.so for Cg hardware shaders.

• Second, the actual shader is loaded from files on disk into the graphics board. Files are searched for with the hardware search path configured with the _MI_REG_HARDWARE registry key in the startup file, or the -hardware_path command-line option. The file name must exactly match the shader name, plus a postfix:

  postfix	contents
  _v.cg	NVIDIA Cg vertex shaders
  _f.cg	NVIDIA Cg fragment shaders

  Either one may be missing if the shader does not require this type of shader. (Most shaders do not require a vertex shader and provide only a fragment shader.) For example, for NVIDIA hardware, if the hardware shader name is myphong, mental ray will load the Cg files myphong_v.cg and myphong_f.cg.

• If no such Cg shader can be found, or -hardware native or -hardware fast is specified on the command line to turn off all programmable shaders, mental ray will use plain hardwired OpenGL shading. This requires a C/C++ function that performs the necessary OpenGL calls to load colors etc. This function follows the same conventions as the support functions in the first item: the shaders get the prefix migl_, for example migl_myphong, and they are loaded from the library migl_base.so. The only difference is that they are loaded instead of, rather than in addition to, any hardware shaders.

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